1 – Your muscles in your back have to work harder to carry the extra load.
2 – As your muscles get tired, you tend to lean backward to compensate. This backward compensatory lean compresses the joints in your lower back which tends to cause pain in a band across the lower back.
There is not much you can do about this pain apart from gentle exercise such as yoga, aqua aerobics and walking. You’ll probably find that you need frequent rests. Sitting, or perching your backside on something creates flexion in your lower back and tends to relieve this ache
Also, gently therapies like chiropractic and massage can help.